1. Field of the Invention
The present invention relates to a heart stimulator having a pulse generator devised for producing stimulation pulses of varying amplitudes, a lead being intended to be introduced into the heart of a patient and connected to the pulse generator for delivering stimulation pulses to the heart.
2. Description of the Prior Art
To reduce the energy consumption of heart stimulators an automatic threshold search function, a so called AUTOCAPTURE™ function, is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulse. The reason therefor resides in the fact that the evoked response potential is small in amplitude compared to the residual polarization charge. The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds after the stimulation. If the polarization is too high, it could be wrongly interpreted by the evoked response detector as a capture, i.e. contraction of the heart. The AUTOCAPTURE™ algorithm could then by mistake adjust the output amplitude of the stimulation pulse to a value below the actual capture level, which will result in no capture. If the used pacing lead has significant polarization this could consequently disturb the AUTOCAPTURE™ function and result in loss of capture.
Several attempts have been made to solve the lead polarization problems in connection with evoked response detection. One possibility is to use low polarization leads. This is, however, not always possible.
Another method is described in U.S. Pat. No. 5,417,718, which discloses a system for maintaining capture wherein electrical post-stimulus signal of the heart, following delivery of a stimulation pulse, is compared to a polarization template, determined during a capture verification test. A prescribed difference between the polarization template and the post-stimulus signal indicates capture. Otherwise loss of capture is presumed and the stimulation energy is increased a predetermined amount to obtain capture.
In U.S. Pat. No. 5,697,957 a method and an apparatus for extracting an evoked response component from a sensed cardiac signal by suppressing electrode polarization are descsribed. An autocorrelation function is then calculated according to an autocorrelation algorithm, and is applied to the sensed cardiac signal. The autocorrelated signal thus obtained and the sensed cardiac signal are then normalized to each other and a difference between these two normalized signals is formed, thereby extracting the evoked response component if present in the cardiac signal.
In U.S. Pat. No. 5,741,312 a method and an apparatus are described to determine stimulating threshold through delivery of pulse pairs consisting of a first lower amplitude search pulse with variable amplitude and a second regular pacing pulse within 50–100 ms. Threshold search is executed by incrementing the amplitude of the search pulse until an evoked response is detected. Alternatively the period from regular pacing pulse to the T-wave is measured and capture on the search pulse is determined as a sudden shortening of this interval. U.S. Pat. No. 5,741,312 further discusses methods to minimize polarization by optimizing pulse paramers of a two- or triphasic pacing pulse.
There is mostly at least one significant slope in the bipolar measured IEGM signal, which makes it possible to discriminate the evoked response signal from slowly varying signals such as polarization signals. Thus in U.S. Pat. No. 5,431,693 a method of verifying capture of the heart by a cardiac pacemaker is described. Observing that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small-amplitude perturbations superimposed on the exponential wave form, these perturbations are enhanced for ease of detection by processing the wave form signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked reponse detection.
Unipolar detection of evoked response signals is however not possible by this technique. Abrupt slope changes or superimposed small-amplitude perturbations are levelled out if the measurements are made over a longer distance from the electrode to the stimulator casing.
This is illustrated in FIG. 1 herein, which shows the unfiltered measured electrode signal picked up by a unipolar electrode configuration, the upper curve in the figure, and a bipolar electrode configuration, the lower curve in FIG. 1.
In co-pending United States Application filed simultaneously herewith and identified with International patent application No. PCT/SE99/01017) a new technique is described for solving the polarization problem in connection with evoked response detection. This technique is not based on any slope measurements on the sensed electrode signal, but on determination of the polarization signal for different stimulation amplitudes for then subtracting the polarization signal from the sensed electrode signal to obtain the true evoked response signal. This determination of the polarization is based on the observations that the evoked response signal amplitude is fairly constant, independent of the stimulation pulse amplitude, whereas the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration, cf. European Application 0906768.
The above mentioned manner of determining the polarization presumes that the stimulation threshold value is known.